It is known in the ophthalmological field that oxygen from the air must be made available to the eye in order to provide for the physiological needs of the cornea. There is also the concomitant need for the carbon dioxide produced by corneal metabolic processes to be carried away from the cornea. The placement of a gas impermeable contact lens over the cornea can seriously inhibit the transmission of oxygen to it and result in corneal trauma. This situation has been partially alleviated by the so-called "pump" design of corneal contact lenses which serves to replace the lachrymal fluid under the lens containing carbon dioxide with freshly oxygenated lachrymal fluid and thereby make oxygen available to the cornea. The oxygen deficiency problem has been further alleviated by limiting the length of time for which an impermeable lens can be continuously worn, ranging from about four hours to 16 hours of daytime wear depending on the individual, and no wear during sleeping hours.
It is now known that hydrophilic contact lenses have gas permeability orders of magnitude greater than conventional hard polymethyl methacrylate lenses and, furthermore, the gas permeability generally increases with water content. The term "gas permeability" refers broadly to air, oxygen and carbon dioxide permeability. Thus, hydrophilic contact lenses with high water content are especially desirable. At sufficiently high water contents the increased gas permeability makes possible increased wearing times, even during sleeping hours. The practical use of hydrophilic contact lenses of high water content has, however, been severely limited hitherto because the strength (resistance to tearing, puncturing, etc.) of contact lenses made from high water content polymers, e.g., 70%-95% water, have been found to be low. Strength has been found to decrease progressively with increased water content. Polymers of the prior art which are of sufficiently high water content to give high gas permeabilities, e.g., about 60%-95% by weight of the combined weight of polymer plus water, and especially above about 70% water content, are very weak and are readily torn or otherwise physically damaged during handling. Such polymers are exemplified in British Pat. No. 1,391,438 and U.S. Pat. Nos. 3,639,524 and 3,943,045. These polymers are prepared from monomer compositions which contain a relatively high amount of cross-linking monomer, the latter being required to prevent the hydrophilic polymer from substantially dissolving in aqueous media. This excessive cross-linking frequently results in a weak polymer. Such high water content hydrophilic polymers are sometimes so fragile that contact lenses made therefrom can only be inserted and removed by a professional practitioner.
Further, the high water content lenses of the prior art cannot withstand repeated heat disinfection, heat sterilization, or cleaning without deterioration or destruction of the lens.
It has not been possible heretofore to have hydrophilic polymers from which strong hydrophilic gel contact lenses of high water content and gas permeability can be made, which lenses can be repeatedly cleaned, disinfected, or sterilized by thermal means or by chemical means without damage to their optical or physical properties.